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Antiphospholipid antibodies (aPL) are a relevant serological indicator of antiphospholipid syndrome (APS). A solid-state surface with covalently bound ω-amine-functionalized cardiolipin was established and the binding of β2-glycoprotein I (β2-GPI) was investigated either by use of surface plasmon resonance (SPR) biosensor, by electrically switchable DNA interfaces (switchSENSE) and by scanning tunneling microscopy (STM). STM could clearly visualize the attachment of β2-GPI to the cardiolipin surface. Using the switchSENSE sensor, β2-GPI as specific ligand could be identified by increased hydrodynamic friction. The binding of anti-cardiolipin antibodies (aCL) was detected against the ω-amine-functionalized cardiolipin-modified SPR biosensor (aCL biosensor) using sera from healthy donors, APS patients and syphilis patients. Our results showed that the aCL biosensor is a much more sensitive diagnostic device for APS patients compared to previous methods. The specificity between β2-GPI-dependent autoimmune- and β2-GPI-independent infection-associated types of aPLs was also studied and they can be distinguished by the different binding kinetics and patterns.

We describe the behavior of 72mer oligonucleotides that are end-tethered to gold surfaces under the influence of applied electric fields. The DNA extension is measured by fluorescence energy transfer as a function of the DNA hybridization state (single- and double-stranded), the concentration of monovalent salt in solution (100 microM to 1 M NaCl), the applied electrode potential (-0.6 to +0.1 V vs Pt), and the temperature (1 to 50 degrees C). At high ionic strength, the DNA conformations are very robust and independent of the applied electrode potential and temperature variations. In solutions of medium ionic strength, the DNA conformation can be manipulated efficiently by applying bias potentials to the Au electrodes. The molecules are repelled at negative potentials and attracted to the surface at positive potentials. The conformation transition occurs abruptly when the electrode bias is swept by merely 0.1 V across the transition potential, which shifts negatively when the salinity is decreased. The behavior can be understood by electrostatic screening arguments and, in the case of single-stranded DNA, when secondary structures are taken into account. At low ionic strength, the experiments reveal an intriguing temperature-dependent stiffening of single-stranded DNA, which can be rationalized by combining counterion condensation theory with the Odjik-Skolnick-Fixman description of the electrostatic persistence length and the unstacking of bases at elevated temperatures.

DNA oligo-nucleotides, localized at Au metal electrodes in aqueous solution, are found to be released when applying a negative bias voltage to the electrode. The release was confirmed by monitoring the intensity of the fluorescence of cyanine dyes (Cy3) linked to the 5' end of the DNA. The threshold voltage of the release changes depending on the kind of linker added to the DNA 3'-terminal. The amount of released DNA depends on the duration of the voltage pulse. Using this technique, we can retain DNA at Au electrodes or Au needles, and release the desired amount of DNA at a precise location in a target. The results suggest that DNA injection into living cells is possible with this method., ((c) 2004 American Institute of Physics)

Abstract: We report on a method to control the packing density of sulfur-bound oligonucleotide layers on metal electrodes by electrical means. In a first step, a dense nucleic acid layer is deposited by self-assembly from solution; in a second step, defined fractions of DNA molecules are released from the surface by applying a series of negative voltage cycles. Systematic investigations of the influence of the applied electrode potentials and oligonucleotide length allow us to identify a sharp desorption onset at −0.65V versus Ag/AgCl, which is independent of the DNA length. Moreover, our results clearly show the pronounced influence of competitive adsorbents in solution on the desorption behavior, which can prevent the re-adsorption of released DNA molecules, thereby enhancing the desorption efficiency. The method is fully bio-compatible and can be employed to improve the functionality of DNA layers. This is demonstrated in hybridization experiments revealing almost perfect yields for electrically “diluted” DNA layers. The proposed control method is extremely beneficial to the field of DNA-based sensors. [Copyright &y& Elsevier]